It has been known that many heavy metals, such as lead, arsenic, and selenium, are toxic to humans even at low levels. One cause for the presence of these heavy metals in the environment has been increasing industrial activities in the recent past. However, in some parts of the world, high levels of heavy metals, such as arsenic, exist naturally in underground water sources because of natural occurrence of these metals in rock formations. Recent epidemiological studies on the carcinogenicity of arsenic have triggered increasing concern about the concentration of arsenic in drinking water and have prompted reevaluation of the current United States maximum contaminant level (“MCL”) of 50 μg/l for arsenic, which will be lowered to 10 μg/l in January 2006. Some recent studies on long-term human exposure show that arsenic in drinking water can be associated with liver, lung, kidney, and bladder cancer. Over exposure to selenium has been shown to have undesired effects on the nervous system and to contribute to the cause dyspnea, bronchitis, and gastrointestinal disturbance.
Many experimental techniques have been proposed or tested for removing arsenic. All of these techniques have achieved varying degrees of effectiveness when arsenic is first oxidized to As(V). Coagulation using alum or ferric sulfate has been shown to have an effect on arsenic levels at a near neutral pH in laboratory and pilot-plant tests. However, the efficiency of this process decreases sharply at low or high pHs. Moreover, the coagulant containing arsenic must be filtered, resulting in additional costs. Lime softening techniques have been shown to be effective at pH levels greater than about 10.5; and, therefore, is not likely to be applicable in drinking water applications. Adsorption treatment methods using activated alumina or ion exchange have been proposed and tested on a pilot-plant scale. However, adsorption of arsenic on alumina is seriously compromised when other ions are present, such as selenium, fluoride, chloride, and sulfate. The adsorption process using ion exchange adsorbents can remove arsenic, but sulfate, total dissolved solids (“TDS”), selenium, fluoride, and nitrate also compete with arsenic for the ion exchange capacity, thus decreasing likely effectiveness.
Therefore, there is a need to provide simple and convenient materials and methods for removing heavy metals such as arsenic and/or selenium from the environment that do not have the disadvantages of the prior-art materials and methods. It is also desirable to provide convenient materials and methods for removing arsenic and/or selenium from the environment, which materials and methods can be made widely available at low cost.